1. Field of the Invention
The present invention relates to a drive unit for a liquid ejection head for ejecting a liquid such as ink and the like by controlling the voltage applied to piezoelectric elements. In particular, the present invention relates to a drive unit that adjusts residual polarization of piezoelectric elements while no liquid ejection operation is conducted and minimizes difference between the elements. Further, the present invention also relates to a liquid ejection apparatus such as a printer that is equipped with such a drive unit, and to a drive method for a liquid ejection head.
2. Description of the Related Art
Ink-jet recording heads of an on-demand type comprise pressure chambers in which ink pressure is generated by piezoelectric elements or heat-generating elements, ink chambers supplying the ink into the pressure chambers, and nozzles ejecting the ink from the pressure chambers. Pressure is generated by applying drive signals to the elements corresponding to the printing signals, and ink droplets are ejected from the nozzles onto the recording medium. In particular, the advantage of the ink-jet recording heads using piezoelectric elements over the ink-jet recording heads of other types is that because no heat is used, the degradation of ink and clogging are prevented.
It is well known that in the ink-jet recording heads using piezoelectric elements, the piezoelectric film is subjected to polarization treatment in advance in order to improve the ejection characteristic of ink by the piezoelectric film.
The relationship between strain (S) and electric field (E) in a piezoelectric film is shown in FIG. 7 to explain a concept of polarization treatment. When no polarization treatment is conducted, strain S=0 if the electric field E=0. If the piezoelectric element is driven starting from this state, the strain S rises along the solid line L as the electric field E increases. On the other hand, when the polarization treatment was conducted in advance, the strain S is already above zero because of the polarization if the electric field E=0. If the piezoelectric element is driven starting from this state, the strain S rises along the solid line H as the electric field E increases. Thus, even when the same electric field is applied from the electric field E=0, the higher strain can be obtained with the polarization treatment conducted in advance than without it.
The polarization produced by such polarization treatment is gradually lost with the passage of time. Japanese Patent Laid-open Publication No. 9-141866 discloses re-polarization of a piezoelectric element member by a voltage having the same polarization direction as in an ink ejection. As a result, ink can be ejected in the desired ejection amount even after long-term usage.
The above-described polarization treatment was effective when the head is driven within a range below the coercive electric field. However, if a piezoelectric thin film is used, the electric field for driving is sufficiently higher than the coercive electric field. As a result, the polarization treatment does not fully demonstrate its effect. On the other hand, the piezoelectric thin films tend to loose the residual polarization faster. For this reason, a polarization of an element having a drive history becomes higher than another element not having a drive history, causing a difference between the elements.
Accordingly, it is an object of the present invention to provide a drive unit for a liquid ejection head, which is capable of suppressing the difference in displacement between the piezoelectric elements.